Jack-ups have been used for oil or gas well drilling, work platforms, oil or gas production platforms, and many other uses. These jack-ups usually consist of a barge shaped hull, generally triangular in plan, supported by three or more trussed legs which usually extend vertically through openings in the hull at the “corners” of the triangle, or extremities of the hull. The trussed legs are usually fitted with vertically extending toothed gear racks on the chords of the legs and the hull is usually fitted with elevating gear units, commonly referred to as “jacks”, that engage with the gear racks to raise and lower the legs when the jack-up is afloat and to raise and lower the hull when the legs have penetrated the ocean floor.
For normal operations, when putting a jack-up on an operating location, the legs are lowered to the ocean floor with the jacks, and jacking continues until soil resistance to penetration of the legs causes the hull to lift out of the water a few feet. Additional soil resistance is usually developed to simulate the largest reaction between the legs and the ocean floor that may be anticipated while at that location. This is normally done by pumping sea water into ballast compartments of the hull.
After developing this additional soil resistance, the hull is then elevated to the desired elevation, which is at least high enough to assure that the crest of the largest anticipated waves will be below the bottom of the hull.
While elevated in this operating position, jack-ups may be subjected to large forces from storm winds, waves and currents. These forces induce stresses in the hull and trussed leg members. Resistance to these storm induced stresses normally determines the strength requirements for the design of the leg members. Additionally, resistance to these storm forces usually determines the maximum leg footing reaction to the ocean floor, therefore it also determines the preload tank capacity requirements. These forces induce large interacting forces and moments between the hull and the legs of jack-ups. For jack-ups without leg-to-hull locking systems, resisting these interacting forces may control the design of the leg guide support structure and portions of the elevating gear units. For jack-ups with leg-to-hull locking systems, resisting these interacting forces usually controls the design of the leg-to-hull locking systems and their support structure on the hull.
The elevating gear units of a jack-up, commonly referred to as “jacks”, are usually mounted in housings that are located radially out from the center of each leg chord and extend vertically up from a location above the top deck of the hull. The gear units are normally mounted one above the other in the housings. Usually there are two levels of leg guides which keep the legs relatively perpendicular to the hull bottom. With this arrangement, the jacks resist all vertical interaction forces between the hull and the legs, and the jacks work together with the leg guides to resist the storm induced moment between the hull and the legs. Some jack-ups have hull-to-leg locking systems, commonly referred to as “rack chocks”, that are installed after the jack-up is elevated to the operating position. These locking systems are used to support the hull on the legs and resist the interacting forces between the hull and legs that are caused by the environmental forces.
U.S. Pat. No. 5,906,457, issued to the applicant, is illustrative.
Recently the exploration and production in deeper water locations has become increasingly important. Available existing jack-ups are often not suited to deeper water, or more sever conditions, or the combination of water depth and environmental criteria of the desired location. The large loads from storm winds, waves and currents, combined with longer leg lengths cause studies for using existing jack-ups to show that one or more of the above limiting design parameters is exceeded. The prior art solution has been to use floating rigs at greater cost.
U.S. Pat. No. 4,378,178 to Roach relates to a lightweight offshore platform structure for use at a plurality of successive sites which is adjustable in height to accommodate a range of water depths. A plurality of anchors spaced around the structure and attached to the ocean floor comprise an anchoring means and are joined to the structure via lower and upper guylines. The attachment of the lower guylines occurs below the water surface sufficiently deep so as to avoid interfering with boats and the like, whereas the upper guylines are adjustable attached the platform to stabilize the structure against storm conditions.
U.S. Pat. No. 3,515,084 to Holmes discloses a floatation unit which may be added to a conventional mat jack-up type platform to permit use of the drill platform in both shallow and deep water drilling operations. As shown in FIG. 2, the entire apparatus is anchored to the seabed by a plurality of mooring lines, symmetrically arranged about the platform.
U.S. Pat. No. 4,797,033 to Polack shows an anchor line-stabilized system for an articulated tower system including at least three chain devices or lines having upper ends coupled to an upper portion of the tower and lower ends anchored to the sea at locations spaced about the tower. Inclinometer means are utilized to sense tilting of the tower and to operate winch means that pull on at least one chain device extending largely opposite to the direction of tilting.
U.S. Pat. No. 4,818,146 to Fontenot provides a stabilizer for an offshore wellhead and conductor comprising an annular braced secured to the conductor, the brace including a plurality of pulleys symmetrically disposed around the brace below the surface of the water. A plurality of cables is each secured at a first end of the cable to the brace, and each of the cables is journaled around one pulley and extends outwardly and downwardly from the pulleys of the brace down to the mudline. The cable is secured at its second end to an anchor pile beneath the mudline for holding the cables in a fixed position. Similar systems incorporating an annular brace as well as a plurality of anchoring cables are shown in U.S. Pat. Nos. 4,710,061 and 4,640,647 both to Blair et al. A related system of anchoring cables but lacking an annular brace is shown in U.S. Pat. No. 5,061,131 to Petty et al.
U.S. Pat. Nos. 5,906,457, 4,378,178 to Roach, U.S. Pat. No. 3,515,084 to Holmes, U.S. Pat. No. 4,797,033 to Polack, U.S. Pat. No. 4,818,146 to Fontenot, U.S. Pat. No. 4,710,061 and U.S. Pat. No. 4,640,647 both to Blair et al., and U.S. Pat. No. 5,061,131 to Petty et al. are all hereby incorporated by reference. The prior art does not provide a solution for increasing the service life, deeper water capability, or more severe environmental capacity for existing jack-up rigs.